1. Field of the Invention
The present invention relates to a ceramic dielectric composition having a high dielectric constant and a small variation of its dielectric constant with temperature and which is easily formed into excellent ceramic devices.
2. Description of the Prior Art
Heretofore, it has been known that ceramic dielectric compositions having small temperature variations with changing temperature can be prepared by adding a bismuth compound to a dielectric, e.g., bismuth stannate is added to barium titanate to provide flat temperature characteristics in U.S. Pat. No. 2,908,579 and U.S. Pat. No. 3,410,705. In such compositions, a grain boundary is formed by the coating of the crystals of BaTiO.sub.3 with a bismuth compound having a low melting point. As a result, the Curie point of the BaTiO.sub.3 is changed such that a dielectric constant rather insensitive to temperature changes ensues.
In the past, there has been a need for a small ceramic capacitor having a large capacity and excellent characteristics for high frequency applications. The known compositions have not been satisfactory for such uses. It is advantageous for such a small ceramic capacitor with large capacity to have a high dielectric constant. However, a ceramic capacitor having a high dielectric constant usually has large variations of dielectric constant with temperature changes so that the size and the capacity have been limited.
Accordingly, in practice there have been attempts to decrease the thickness of the dielectric substrate and to increase its capacity. Dielectric substrates with thicknesses of 0.1 - 0.2 mm have been obtained. In the conventional technique of incorporating a bismuth compound in the composition, are several significant disadvantages: the bismuth compound can become vaporized; the substrate can become bent; or pin-holes can be formed making it difficult to produce a ceramic having high density. This is a deleterious factor for determining the maximum voltage of the capacitor. As a result, incorporation of an additive which is not vaporized by calcination is required. Additionally, the composition containing the bismuth compound has a disadvantageously high loss in high frequency operation. Moreover, bismuth is a toxic heavy metal so that use of a bismuth compound poses a serious problem from the viewpoint of heavy metal pollution.
Ceramic dielectric compositions having the formula BaTiO.sub.3 -Nb.sub.2 O.sub.5 -MgO are disclosed in U.S. Pat. No. 3,788,867 and Ceramic Bulletin Vol. 50, no. 5, pages 488-492 (1971). These compositions have .epsilon..sub.s values of less than 2000 and inferior temperature characteristics. When MgO of the composition is replaced by CeO.sub.2, the .epsilon..sub.s values of the compositions are about two times (about 4000) that of the conventional compositions.
As can be seen, it would be most desirable to have a dielectric ceramic composition which has a high dielectric constant which is insensitive to temperature variations.